Laser-induced breakdown spectroscopy (LIBS) is an emerging analytical technique that offers advantages over established techniques for certain types of analysis. It can analyze materials without any sample preparation, and because it consumes only a tiny amount of material, it is considered virtually nondestructive. LIBS also is very fast, and can be used for remote analysis of up to 10–100 m for many applications.
The versatility of LIBS is currently being explored, and the technique has been used in many different applications, such as forensics, manufacturing process control, and even space exploration. One developing application area for LIBS is the characterization of nuclear material. LIBS is already being used at nuclear energy sites in the United Kingdom. So could LIBS help Japan with its current nuclear crisis? To find out, we spoke to Andy Whitehouse, managing director of Applied Photonics Ltd (Skipton, North Yorkshire, UK).
Spectroscopy: Could LIBS be useful to detect contamination from the recent nuclear accident at the Fukushima Daiichi power plant in Japan?
Whitehouse: I believe so. As a material characterization technique, LIBS is not in direct competition with conventional nuclear measurement techniques such as gamma-ray spectroscopy for most cases (such as for the detection of specific radionuclides like cesium-137), but LIBS could offer unique capabilities in certain areas where conventional techniques are unsuitable or are otherwise unable to provide useful information.
One such application scenario would be the remote and in-situ detection of uranium or mixed oxide (MOX) fuel, damaged or otherwise, located in hard-to-reach places such as inside the reactor pressure vessel, or submerged in water or sludge. Gamma spectroscopy would struggle to do this for a number of reasons, the prime reason being that gamma spectroscopy cannot easily detect uranium or plutonium because of the low gamma yield of these materials, especially in an environment that has a high gamma field as a result of the presence of fission products, such as cesium-137, either within the uranium or plutonium fuel or dispersed in the form of contamination. LIBS is essentially immune to the effects of ionizing radiation (gamma, neutron, etc.), so LIBS can, and has been, deployed in environments of extreme levels of radiation such as within a high-level waste vitrification plant.
Spectroscopy: Could a standard LIBS instrument be used in such an environment?
Whitehouse: Yes, but in my opinion it would be the fiber-optic probe version of our LIBS technology that would have the most potential at Fukushima since it is able to operate in limited-access and high radiation areas and can also be used to identify materials submerged in water or sludge. The use of seawater to cool the overheating nuclear fuel at Fukushima, both within the reactor pressure vessel and the spent-fuel storage pond, will, I understand, result in the formation and accumulation of a sludge-like material as the seawater evaporates. This will make future inspection of the damaged reactor components and fuel considerably more difficult. Should there be a need to pinpoint the location of the melted nuclear fuel within the damaged structure, then our submersible LIBS probe could provide a unique solution.
Spectroscopy: Has LIBS ever been used before at a nuclear accident site?
Whitehouse: About 12 years ago, a team of scientists and engineers from Chernobyl visited us to learn more about our fiber-optic LIBS probe with a view to using it at Chernobyl to identify the location of fragments of melted nuclear fuel (primarily uranium). At the time, that was important to them because removal of the fuel from the wreckage of the reactor would significantly reduce the overall radioactivity content of the damaged structure, because most of the radioactivity is tied up in the fuel. In the end, however, a decision was made to leave the fuel inside the damaged structure. I believe, however, that the latest plans are to go back and remove the fuel before replacing the temporary confinement structure that was built over the damaged reactor to prevent the spread of radioactive contamination into the environment, because the confinement structure is now in an unsafe condition. Of course, the LIBS technology, whether standoff or fiber optic, has improved considerably over the intervening 12 years.
Spectroscopy: Can standoff LIBS be used to identify nuclear residues at a distance? If so, at what standoff distance can this be done?
Whitehouse: We have designed and built standoff LIBS instruments with a range capability in excess of 100 m for the defense market. We have used a transportable standoff LIBS instrument with 20-m range capability at the Sellafield nuclear site in Cumbria, UK, where it was used to characterize radioactive materials within a hot cell by directing the laser beam through a lead-glass shield window. I believe we were the first to deploy a LIBS instrument in this way.
Spectroscopy: Would LIBS on a robot be useful in such a scenario?
Whitehouse:For some applications, the deployment of our fiber-optic LIBS probe requires some form of remote manipulator or robotic device in order to position the probe where required. In the UK nuclear industry, we have used so-called master-slave manipulators to deploy a LIBS probe within a process cell at the Sellafield high-level waste vitrification plant. In other applications, all that is necessary is a simple rod or pole which may be used by hand to position the LIBS probe where required — for example within a storage pond.
Spectroscopy: Would LIBS be useful for detecting nuclear contamination in matrices such as soil, water used at a nuclear plant, seawater, plants, fish?
Whitehouse: The application scenarios you have listed would best be served by conventional nuclear measurement techniques (gamma spectroscopy, alpha spectroscopy, neutron detection techniques) because these techniques are orders of magnitude more sensitive than LIBS and so would be appropriate for detecting low and medium levels of radioactivity in the environment. LIBS would be better suited to applications where radiation levels are very high or where physical access is limited.
Spectroscopy: For most elements, LIBS cannot distinguish between stable and radioactive isotopes of the same material; rather, it can detect the presence of an element associated with nuclear radiation, such as plutonium. How does that affect how you would apply LIBS to analyzing radioactive contamination?
Whitehouse: To be correct, LIBS can in principle detect specific isotopes assuming a suitably high-resolution optical spectrograph is used, although this would perhaps not be its most useful feature in the given application since there are many other techniques which offer higher sensitivity. LIBS has been used to distinguish different isotopes of various elements, including uranium (235 and 238), plutonium (239 and 241) and isotopes of certain lighter elements such as lithium. To the best of my knowledge, LIBS has not been used to detect cesium-137 — a common and abundant fission product in irradiated nuclear fuel — most probably because the specific activity of cesium-137 is so high that only a vanishingly small quantity of this radionuclide would be needed to be radiologically significant (i.e., potentially harmful) and LIBS would not be able to detect such vanishingly small quantities of material. In general, radiologically significant quantities of radionuclides of relatively high specific activity (such as cesium-137, strontium-90, and iodine-129) are best monitored and detected using conventional nuclear measurement techniques. On the other hand, radiologically significant quantities of radionuclides of relatively low specific activity (such as uranium-235, uranium -238, plutonium-239, plutonium-241, and technetium-99) may be amenable to monitoring and detection by LIBS. In particular, their presence could be confirmed by LIBS in cases where conventional nuclear measurement techniques are not appropriate or practical, such as in material submerged in water or sludge, in limited-access environments, or in high gamma field environments, as mentioned earlier.
Spectroscopy: Can LIBS provide quantitative analysis of radioactive elements?
Whitehouse: For the given application, my view is that LIBS is best suited to detecting the presence of a specific material, such as uranium fuel fragments submerged in seawater sludge, rather than measuring the quantity of the material present — the latter being totally impractical for the type of scenarios I envisage at the Fukushima plant.
Spectroscopy: Could LIBS be useful for a possible nuclear crisis at a site where they use nuclear power plants of a type different from those used at Fukushima?
Whitehouse: Possibly, but I believe that LIBS has significantly more potential in certain areas of nuclear decommissioning where huge quantities of radioactive waste of largely unknown inventory exists, such as at redundant nuclear processing facilities (both military and civilian), redundant spent-fuel storage ponds, and temporary radioactive waste storage facilities.
One only has to make a cursory review of the open literature on some of the redundant facilities in the UK (such as Sellafield) and the USA (such as the Hanford site) to understand the magnitude of the problem of characterizing the huge quantities of miscellaneous radioactive waste that are currently stored under less-than-ideal conditions. The Fukushima plant may now be regarded as a nuclear decommissioning site since it seems highly unlikely that it will operate again and the mess will have to be cleaned up sooner rather than later.
Spectroscopy: Can you tell us more about how LIBS is now being used in the UK’s nuclear energy industry?
Whitehouse: My company has used LIBS in the UK nuclear industry for a variety of applications within operational plants. Examples include the identification of advanced gas-cooled reactor (AGR) superheater tubes manufactured from a rogue batch of stainless steel, determining the chromium content of economizer tubes in an AGR steam generator as part of an erosion-corrosion inspection, and remote characterization of high-level radioactive waste at a nuclear reprocessing plant.
For more than 10 years, we have been promoting LIBS technology for nuclear decommissioning applications, but despite our best efforts we have not yet undertaken a real-world application. During the last year, we have received an increased level of interest from this sector, but we have come to accept that things move very slowly in nuclear decommissioning, especially where uptake of new technology is concerned. We have not given up hope though!